1) Field of the Invention
The present invention relates to an image forming apparatus such as a facsimile, a printer, a copying machine, or a multifunction product and, more particularly, to an image forming apparatus which transfers a visual image on an image carrier to a movable object side such as a conveyer belt or an intermediate transfer belt at a counter position between the image carrier and the movable object.
2) Description of the Related Art
In recent years, a color image forming apparatus that forms a high-quality image has gained popularity both at home and abroad. In particular, a tandem type color image forming apparatus, in which a plurality of image forming units are arranged along a conveyer unit to realize an image forming process at a high speed, is popular.
In the tandem type color image forming apparatus, optical beams emitted from a plurality of light sources are irradiated on the plurality of image carriers arranged in the apparatus, to form an electrostatic latent image. Developing agents having different colors (for example, toners of three colors: yellow (Y), magenta (M), and cyan (C), or of the three colors and black (Bk)) are caused to adhere to the electrostatic latent images, to form toner images that serve as real images. Thereafter, a recording material such as recording paper or the like, carried on the movable object such as a conveyer belt, is sequentially conveyed to transfer positions of the image carriers. The toner images are superimposed on the recording material to transfer the image. The transferred toner image is fixed on the recording material to form a multi-color image.
In the color image forming apparatus, the toner image is transferred at a high speed, onto the recording material such as a recording paper moving in the conveyance direction. Feedback control may be performed with respect to the moving speed of the movable body, to stabilize positioning accuracy of the respective colors.
Japanese Unexamined Patent Publication No. H11-146675 discloses an image forming apparatus that determines an error when a speed error of the conveyer belt drive motor or a positioning error exceeds a tolerance value, and stops photoconductor members and a transfer belt.
Japanese Unexamined Patent Publication No. H11-24507 discloses an image forming apparatus that performs drive source control based on results obtained by reading scales formed on the rotating member (transfer belt), and performs feedback control of the speed of the transfer belt, to position images on the transfer member with high accuracy.
However, due to the following problems in execution of the feedback control, an erroneous image may be output.
As the first problem, when a toothless output pulse is generated from an out-of-order encoder, a decrease in speed or lack of a position forward distance is detected. Thus, when the feedback control is performed in this state, the rotating speed of a motor that rotationally drives a movable member increases excessively.
As the second problem, when noise is superimposed on an encoder output, it is detected that a speed increases or that a position forward distance is excessive. Hence, when the feedback control is performed in this state, the rotating speed of the motor that rotationally drives the movable member reduces excessively.
As the third problem, when movement of the movable member suddenly varies, the rotating speed of the motor that rotationally drives the movable member excessively increases or reduces. Depending on circumstances, control oscillation can occur.
In Japanese Unexamined Patent Publication No. H11-146675 stated above, when a control error is detected by comparing the speed error tolerance with the positioning error tolerance, the photoconductor members and the transfer belt are stopped.
However, the problems are unique to when the feedback control is performed. If the feedback control is not performed, though color shift may worsen slightly, a user can obtain an image having practically no problem. Therefore, prohibiting image formation without reason results in downtime of the image forming apparatus, which some users do not like.